1. Field of the Invention
This invention relates to a kinematic optical mounting to support an optical mounting. More particularly, this invention relates to a kinematic optical mounting to support a lens cell in an optical barrel such as the optical barrel used in the projection lens system for a photolithography process to manufacture semiconductor wafers, or in a photographic camera.
2. Description of the Related Art
In manufacturing integrated circuits using photolithography, light is transmitted through non-opaque portions of a pattern on a reticle, or photomask, through a projection exposure apparatus, and onto a wafer of specially-coated silicon or other semiconductor material. The uncovered portions of the coating, that are exposed to light, are cured. The uncured coating is then removed by an acid bath. Thus, the layer of uncovered silicon is altered to produce one layer of the multi-layered integrated circuit. Conventional systems use visible and ultraviolet light for this process.
As the miniaturization of a circuit pattern progresses, the focus depth of the projection exposure apparatus becomes very small, making it difficult to align accurately the overlay of circuit patterns of the multi-layered integrated circuit. As a result, a primary consideration for an overall design of the photolithography system includes building components of the system that achieve precision by maintaining small tolerances. Any vibration, distortion, or misalignment caused by internal, external or environmental disturbances must be kept at minimum. When these disturbances affect an individual part, the focusing properties of the photolithography system are collectively altered.
The projection exposure apparatus commonly includes a lens barrel to carry a plurality of lens serially aligned along an optical axis of the lens barrel. Each lens is mounted on a lens cell. The combination of a lens and a lens cell is generally referred to as the lens cell assembly. In a conventional lens cell assembly, the lens is fastened to the lens cell by chemical adhesives or friction clamping.
The conventional lens cell assembly poses several problems. In a photolithography system where an inert gas, for example nitrogen or helium, is introduced inside the projection exposure apparatus surrounding the lens cell assembly, the adhesive may release gas which could be harmful to the environment of the lens cell assembly. The released gas from adhesive absorbs the exposure light.
One problem with the conventional lens cell assembly utilizing friction clamping is that the clamping force retains or pinches the lens which often causes the lens surface to deform. Moreover, when the projection lens assembly is severely disturbed, such as when the assembly is being shipped from the manufacturer""s site to a production site, the clamping force may cause the lens to break.
In light of the foregoing, there is a need for a kinematic optical mounting and method for kinematically supporting the optical element in the optical mounting assembly that can apply an appropriate amount of force to constrain movement of the optical element without unduly altering the physical and chemical properties of the optical element.
The advantages and purposes of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages and purposes of the invention will be realized and attained by the elements and combinations particularly pointed out in the appended claims.
To attain the advantages and consistent with the principles of the invention, as embodied and broadly described herein, a first aspect of the invention is an optical mounting assembly comprising an optical element, having an optical axis, an outer circumference, and a plurality of mounting pads distributed substantially equi-angularly around the outer circumference. The optical mounting assembly also comprises an inner circumference and an optical holder having a corresponding plurality of clamping brackets distributed around the inner circumference. The optical holder supports the optical element at points of contact between the respective mounting pads and the corresponding plurality of clamping brackets.
Another aspect of the present invention is an optical mounting assembly comprising an optical element having an optical axis, and an optical holder supporting the optical element at three points of contact distributed equi-angularly around the optical holder. The optical element is constrained by the optical holder in six degrees of freedom at the three points of contact, three degrees of freedom in a normal direction parallel to the optical axis, and another three degrees of freedom in a tangential direction of the optical holder at the corresponding three points of contact.
A further aspect of the present invention is a method for making an optical mounting assembly having an optical holder supporting an optical element. The method comprises the steps of forming a plurality of mounting pads distributed substantially equi-angularly around an outer circumference of the optical element, and supporting the plurality of mounting pads by a corresponding plurality of clamping brackets distributed around an inner circumference of the optical holder for supporting the optical element at points of contact between the plurality of mounting pads on the optical element and the corresponding plurality of clamping brackets on the optical holder.
Yet a further aspect of the present invention is a method for supporting an optical element in an optical mounting assembly. The optical mounting assembly has an optical holder for supporting the optical element and has an optical axis. The method comprises the step of providing three points of contact distributed substantially equi-angularly between an inner circumference of the optical element and an outer circumference of the optical holder. The method also comprises the steps of constraining the optical element to the optical holder in three degrees of freedom in a normal direction parallel to the optical axis at the corresponding three points of contact, and constraining the optical element to the optical holder in other three degrees of freedom in a tangential direction of the optical holder at the corresponding three points of contact.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. Additional advantages will be set forth in the description which follows, and in part will be understood from the description, or may be learned by practice of the invention. The advantages and purposes may be obtained by means of the combinations set forth in the attached claims.